What do you know about electrochemical sensors

The electrochemical sensor is a type of sensor that relies on the electrochemical properties of the analyte to transduce the chemical quantity into an electrical quantity for sensing and detection.

The earliest electrochemical sensors date back to the 1950s, when they were used for oxygen monitoring. And to the 1980s, when they were used to monitor a wide range of toxic gases and showed good sensitivity and selectivity.

Ⅰ. Working principle of electrochemical sensor

 Electrochemical sensors works by reacting chemically with the gas being measured and producing an electrical signal proportional to the gas concentration. Most electrochemical gas sensors are generating current that is linearly proportional to the gas concentration.

 An electrochemical gas sensor works as follows: The target gas molecules in contact with the sensor first pass through a diaphragm that prevents condensation and also acts as a dust barrier. Then the gas molecules diffuse through a capillary tube, possibly through a subsequent filter, and then through a hydrophobic membrane to the surface of the sensing electrode. There the molecules are immediately oxidised or reduced, thus generating or consuming electrons so generating an electric current.

 It is important to note that the amount of gas molecules entering the sensor in this way is limited by diffusion through the capillary. By optimising the path, an appropriate electrical signal is obtained according to the desired measurement range. The design of the sensing electrode is essential to achieve high responsiveness to the target gas and to suppress undesired responses to interfering gases. It involves a three-stage system for solids, liquids and gases, and all involve chemical identification of the analyte gas. The electrochemical cell is completed by the so-called counter electrode, the Cont electrode, which balances the reaction at the sensing electrode. The ionic current between the Cont electrode and the Sen electrode is transported by the electrolyte within the sensor body, while the current path is provided through a wire terminated by a pin connector. A third electrode is usually included in electrochemical sensors (3-electrode sensors). A so-called reference electrode is used to maintain the potential of the sensing electrode at a fixed value. For this purpose and usually for the operation of electrochemical sensors, a constant potential circuit is required.

Ⅱ. Components of an electrochemical sensor

The electrochemical sensor comprises the following four key components:

1. Breathable membranes (also known as hydrophobic membranes): These membranes serve to cover the sensing (catalytic) electrodes and, in certain instances, regulate the molecular weight of gases reaching the electrode surface. Typically, these membranes are fabricated from Teflon films with low porosity. When these membranes are employed to cover the electrodes, the sensors are referred to as coated sensors. Alternatively, a high porosity Teflon film can be utilized, along with a capillary, to control the molecular weight of the gas reaching the electrode's surface. This configuration is known as a capillary type sensor. Besides providing mechanical protection for the sensor, the film also functions as a filter, eliminating unwanted particles. To ensure the appropriate molecular weight of the gas is permitted to pass through, it is crucial to select the appropriate aperture size for both the membrane and the capillary. The aperture size must allow sufficient gas molecules to reach the sensing electrode while preventing leakage or rapid drying of the liquid electrolyte.

2. Electrode: It is crucial to carefully select the electrode material. The material should be catalytic, capable of performing a semi-electrolytic reaction over a prolonged period. Typically, electrodes are crafted from precious metals, like platinum or gold, which react efficiently with gas molecules through catalysis. Depending on the sensor's design, the three electrodes may be constructed from different materials to facilitate the electrolysis reaction.

3. Electrolyte: The electrolyte must be capable of facilitating electrolytic reactions and efficiently transducing ionic charge to the electrode. It must also form a stable reference potential with the reference electrode and be compatible with the materials used within the sensor. Furthermore, rapid evaporation of the electrolyte can lead to a weakening of the sensor signal, potentially compromising its accuracy and reliability.

4. Filters: Occasionally, scrubber filters are positioned in front of the sensor to eliminate undesired gases. The selection of filters is limited, with each type exhibiting a distinct level of efficiency. Activated carbon stands as the most widely utilized filter material, effectively filtering out most chemicals, excluding carbon monoxide. By carefully selecting the appropriate filter media, electrochemical sensors achieve a heightened selectivity towards their intended gases.

Ⅲ. Classification of Electrochemical Sensor

There are many ways to classify electrochemical sensors. Depending on their varying output signals, they can be divided into potentiometric sensors, amperometric sensors, and conductometric sensors.

According to the substances detected by electrochemical sensors, electrochemical sensors can be mainly classified into ion sensors, gas sensors and biosensors.

Ⅳ. Main Properties and Influencing Factors

1. Sensitivity

The main factors that affect sensitivity include: catalyst activity, air intake, electrolyte conductivity, and ambient temperature.

2. Response recovery

The main factors that affect the response recovery speed are catalyst activity, electrolyte conductivity, gas chamber structure, gas properties, etc.

3. Selectivity/Cross-interference

The main factors that affect the selectivity include the type of catalyst, electrolyte, bias voltage, filter, etc.

4. Repeatability/Long-term stability

Factors affecting repeatability include: electrode structure stability, electrolyte stability, gas circuit stability, etc.

5、High and low temperature performance

Factors affecting high and low temperature stability include: catalyst activity, electrode structure stability, and gas characteristics.

V. Four major applications of electrochemical sensors

Electrochemical sensors are widely used in industrial and civilian areas of gas detection, can detect ozone, formaldehyde, carbon monoxide, ammonia, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, oxygen and other gases, commonly used in portable instrumentation and gas online monitoring instrumentation.

1.  Humidity Sensor

Humidity is an important indicator of the air environment, the humidity of the air and the human body has a close relationship between the heat of evaporation, high temperature and high humidity, due to the human body water evaporation difficulties and feel stuffy, low temperature and high humidity, the human body heat dissipation process is intense, easy to cause colds and frostbite. The most suitable temperature for human body is 18~22℃, relative humidity is 35%~65% RH. In the environment and health monitoring, it is commonly used in wet bulb thermo-hygrometer, hand-cranked hygrometer and ventilation hygrometer and other instruments to determine the air humidity.

In recent years, a large number of literature reports on the use of sensors to determine air humidity. The coated piezoelectric quartz crystals used for the determination of relative humidity are made into small quartz piezoelectric crystals by photolithography and chemical etching techniques, and four substances are coated on the AT-cut 10 MHz quartz crystals, which have a high mass sensitivity to humidity. The crystal is a resonator in an oscillating circuit whose frequency varies with mass, and by selecting the appropriate coating, the sensor can be used to determine the relative humidity of different gases. The sensitivity, response linearity, response time, selectivity, hysteresis and lifetime of the sensor depend on the nature of the coating chemicals.

2、Nitrogen oxide sensor

Nitrogen oxide is a variety of oxides of nitrogen composed of a mixture of gases, often expressed as NOX. In the nitrogen oxide, different forms of nitrogen oxide chemical stability is different, the air is often divided into relatively stable chemical properties of nitrogen monoxide and nitrogen dioxide, their significance in hygiene appears to be more important than other forms of nitrogen oxide.

In environmental analysis, nitrogen oxide generally refers to nitrogen dioxide. China's standard method for monitoring nitrogen oxides is the colourimetric method of naphthalene ethylenediamine hydrochloride, the sensitivity of the method is 0.25ug/5ml, the method of conversion coefficient is affected by the composition of the absorbent solution, the concentration of nitrogen dioxide, the speed of gas collection, the structure of the absorber tube, the coexistence of ions and temperature and many other factors, not completely unified. Sensor determination is a new method developed in recent years.

3、Hydrogen Sulfide Gas Sensor

Hydrogen sulfide is a colourless, combustible gas with a special rotten egg smell, which is irritating and asphyxiating, and harmful to the human body. Most methods use calorimetry and gas chromatography to determine hydrogen sulfide in the air. The determination of air pollutants whose content is often as low as the mg/m3 level is one of the main applications of gas sensors, but semiconductor gas sensors are not able to meet the sensitivity and selectivity requirements for monitoring certain pollutant gases in a short period of time.

The silver-doped thin-film sensor array consists of four sensors that simultaneously record the concentrations of sulphur dioxide and hydrogen sulfide using a universal analyzer based on coulometric titration and the signals from the semiconductor gas sensor array. Practice has shown that silver-doped thin-film sensors used at 150 °C in a constant-temperature manner are effective for monitoring hydrogen sulfide content in urban air.

4. Sulfur Dioxide Sensor

Sulfur dioxide is one of the main substances that pollute the air, and detecting sulfur dioxide in the air is a regular part of air testing. The application of sensors in monitoring sulfur dioxide has shown great superiority, from shortening the detection time to lowering the detection limit. Solid polymers are used as ion exchange membranes, with one side of the membrane containing internal electrolytes for the counter and reference electrodes, and a platinum electrode inserted on the other side to form the sulfur dioxide sensor. The sensor is mounted in a flow cell and oxidises sulfur dioxide at a voltage of 0.65V. The sulfur dioxide content is then indicated. The sensing device exhibits high current sensitivity, a short response time, good stability, low background noise, a linear range of 0.2 mmol/L, a detection limit of 8*10-6 mmol/L, and a signal-to-noise ratio of 3.

The sensor can not only detect sulfur dioxide in the air, but also be used to detect sulfur dioxide in low conductivity liquid. The gas-sensitive coating of the organically modified silicate thin film sulfur dioxide gas sensor was fabricated utilizing sol-gel process and spin technology. This coating exhibits excellent reproducibility and reversibility in sulfur dioxide determination, with a swift response time of less than 20 seconds. Additionally, it demonstrates minimal interaction with other gases and is minimally influenced by changes in temperature and humidity.